Some filter factor thoughts/questions...

[mtngael]

Ok, so everything I know about filters for B&W has always indicated that there is no need to compensate exposure with a TTL meter because the meter does it. HOWEVER, all of my TTL meters only indicate a 1/3 loss stop with the filter on, both the TTL and handheld only indicate a 1/3 stop loss with filters that have published filter factors of 1 1/3 and 2 stops, and only a 1 stop loss for a 25A Red filter, which has a published loss of 3 stops. Just to muddy the waters further, I rate my film at an EI of 200 (technically a 1 stop overexposure) and pull development accordingly. Now, I also had info that the filter factors don't come from solely a light compensation issue, but also a film spectral response issue.

I've been using the TTL metered reading as my base in those cameras (prior to making adjustments based on my own tonal values) and a handheld reflected meter for my meterless camera.

The problem is that when using filters (especially a yellow-green) that call for more than 1 stop of compensation I'm getting horribly over-exposed negatives, more so with the meterless camera. With that I've been using the published filter factors and not so with the TTL camera. My development is pulled by a hair under 20% to account for the 1 stop lower EI but I'm still getting horribly blown negatives consistently. The only thing I can think of is that the published filter factors are either horribly wrong OR based on shooting film at box speed, with normal development, and not downrated.

If the true light loss is only 1/3 of a stop, and the film is a stop lower than rated EI, technically I'd already be a stop and 1/3 compensated (not accounting for the reduced development).

So, after all that rambling, can anyone tell me what gives? I'm frustrated because as far as I know I'm doing everything right but losing too many good shots!

 

[Steve Smith]

Your meter doesn't register all colours of light equally. You will need to experiment to work out how much extra you need to add to compensate.

e.g. If you put on an orange filter which has a factor of two and your meter only adds 2/3 of a stop, you will need to tell the camera to compenstae an extra 1 1/3 stops. Once you have done this you should be able to use the same setting every time you use that filter.


Steve.

 

[mtngael]

Your meter doesn't register all colours of light equally. You will need to experiment to work out how much extra you need to add to compensate.

e.g. If you put on an orange filter which has a factor of two and your meter only adds 2/3 of a stop, you will need to tell the camera to compenstae an extra 1 1/3 stops. Once you have done this you should be able to use the same setting every time you use that filter.


Steve.

Ok, that makes sense to me for my metered camera. At least I've done some of the work already!

However, what about the meterless camera. Since I'm exposing with full compensation for the filter factors, not metering with them, why would my negatives all be overexposed so much?

When all I used was a yellow filter, there never seemed to be an issue (probably because it only loses 1 stop!) but now things aren't working so well!

In case it matters, I'm using Hoya HMC filters, and shooting Tri-X and Plus-X. The Tri-X at an EI of 200 and the PX at EI64. The biggest discrepencies I'm seeing are when I use the XO Yellow-Green filter.

Thanks!

 

[Rick A]

I have found that metering at 200 for Trix with a yellow filter gives perfect exposure without adding any extra exposure to it. Same holds true for Plus x and metering at half the listed box speed. I meter Trix at ie 50 with a red filter in bright sun for perfect exposure. I use standard development times with Pyrocat-HD or D-76 (1+1).

 

[Steve Smith]

With a handheld meter I set the ISO to the equivalent with the filter in place. i.e. if I am using ISO 400 film I will set the meter to 200 if I use a yellow filter, 100 for an orange filter and 50 for a red filter.

Obviously if you normally use ISO 400 film at EI 200 (like I do sometimes) then you can halve those figures.


Steve.

 

[jeffreyg]

Have you tried metering through the filters but off a gray card? Possibly that will give you the filter factor for your setup.

http://jeffreyglasser.com/

 

[mtngael]

Have you tried metering through the filters but off a gray card? Possibly that will give you the filter factor for your setup.

http://jeffreyglasser.com/

Jeffery,

I haven't tried that yet, as I was hoping to save the time, effort, and supplies, but now I think I will! I think I'll shoot a with the metered camera, using the ttl recommended exposure and then bracket, and then the same with the meterless camera but bracketing the recommended filter factors.

Steve,

I've been doing exactly as you describe for my metering. EI of 200 with Tri-x and then, if using the XO filter, I set my meter's EI at 50. I cut development time 20% or so, which means I *should* get properly exposed negs. The last ones were way blown.

The only thing I can think of, that just occurred to me, is if I spaced it and used another dilution/time combo for one of the other films when I developed all those rolls. In that case my long rambling question may be moot!

 

[Steve Smith]

What your describing sounds like you are doing everything right... in theory! It might be worth using a roll of film taking a shot of the same subject with various exposures.

Start with a 'normal' scene and take one shot without a filter at you standard EI 200 then do three shots per filter at -2, 0 and +2 stops from what you calculate to be the correct exposure and keep detailed notes.

I wouldn't bother with only one stop of difference as that shouldn't give you the problem you are describing. I also wouldn't bother with rating the film at different speeds at this stage as that just adds extra complication to it.

After processing your standard way (-20% development) you should be able to compare your images. Hopefully the control image without a filter will be correct and if your calculations were right, the middle exposure for each filter should be correct too.

I am assuming that this is for daylight lit subjects, artificial light may have a different response. It also may be possible that your shutter is running slow and causing over-exposure. The first control image on the test film would show this.


Steve.

 

[mtngael]

Thanks Steve, I'll be trying that. You know, the shutter speed issue hadn't even occurred to me! The camera that seems to be having the most effects has a leaf shutter for the lens. When I bought it, the shutter had been left cocked for 6 years. I completely forgot about that as I'd had it for a bit before I got to finally start using it. That may explain everything! Oy...time to get that looked at. Thanks for bringing that to my attention!